This invention relates to liquid crystal devices, such as ferroelectric liquid crystal devices, and methods of manufacturing such devices.
The surface stabilised ferroelectric liquid crystal device (FLCD) possesses the advantage over other liquid crystal devices, such as the twisted nematic liquid crystal device, that it is a bistable device which can be switched between two states by switching pulses of alternate polarity and which will remain in one state in the absence of a switching pulse until a switching pulse of appropriate polarity is applied to switch it to the opposite state. By contrast, in a twisted nematic liquid crystal device, a drive signal must be applied continuously to maintain the device in one of its states.
A conventional FLCD cell comprises a layer of ferroelectric smectic liquid crystal material contained between two parallel glass substrates provided on their inside surfaces with electrode structures in the form of row and column electrode tracks which cross one another to form an addressable matrix array. Furthermore each of the inside surfaces of the substrates is provided with a suitable alignment layer which, prior to assembly of the substrates and filling of the cell with liquid crystal material, is treated by rubbing to impart a preferred surface alignment direction, and preferably a surface pretilt, to the contacting molecules of the liquid crystal material layer.
The switching behaviour of the liquid crystal molecules is dependent on the arrangement of the molecules in microlayers which, in the case of chiral smectic material, extend transversely of the substrates and adopt a chevron geometry having two possible states, C1 and C2, as disclosed in J. Kanbe et al, Ferroelectrics (1991), vol. 114, pp. 3. Both C1 and C2 states can form as the material cools down from the isotropic phase to the chiral smectic phase during device manufacture, and the boundaries between these two states may be seen as a zigzag defect. When used in a display device, material incorporating both the C1 and the C2 states can appear patchy, and it is therefore preferred that the material should be in one state for a practical device. The C2 state is preferred as it allows faster switching at lower voltages. Accordingly it is important that both the alignment layers provided on the substrates have surface alignment properties which are such as to promote formation of the C2 state on cooling of the liquid crystal material layer during manufacture of the device.
However little or no formation of the C2 state may occur with some liquid crystal materials when using a conventional device manufacturing method in which a bath of liquid crystal material is heated to a temperature at which the material is in the isotropic phase, the liquid crystal material is drawn under vacuum between the substrates of the cell, and the cell is then cooled down slowly so that the material passes from the isotropic phase through the cholesteric and smectic A phases to the chiral smectic phase. Furthermore the C2 state may be unstable with temperature so that the proportion of the material in the C2 state may vary with temperature.
In a colour FLCD, such as may be used in a colour display, one of the substrates of the cell may incorporate a colour filter layer incorporating red, green and blue areas for each pixel of the cell. During manufacture of such a device the colour filter layer is applied prior to the application of the alignment layer to the substrate, and this imposes a limit to the temperature of the subsequent heat curing treatment which may be applied to polymerise and harden the alignment layer after spinning down of a liquid monomer on the substrate surface to form the alignment layer in known manner. Whereas the heat curing treatment may take place at a temperature of up to about 300.degree. C. in a cell in which a colour filter layer is not provided, the heat curing treatment must generally take place at a temperature less than 180.degree. C. in a cell in which such a colour filter layer is provided, in order not to adversely affect the colour filter layer. However such a lower temperature heat curing treatment may be insufficient to prevent the surface alignment properties of the alignment layer being significantly changed by heat treatments applied during further processing.
Furthermore spacer walls may be formed on at least one of the substrates for spacing the substrates apart when the substrates are connected together and for securing the substrates together over the entire surface area of the cell. Such spacer walls may be formed by an additional manufacturing step carried out prior to application of the alignment layer to the substrate, the additional manufacturing step typically comprising spinning down of a polyimide layer on the substrate and selective etching of the layer to form the spacer walls at the required locations. Subsequent to the formation of the spacer walls, the alignment layer is applied and rubbed to impart a preferred alignment direction, although the existence of the spacer walls can mean that it is difficult to properly rub all parts of the alignment layer. Furthermore the substrates are connected together by a heat bonding process, at a temperature of 150-180.degree. C. for example, in order to bond the spacer walls on one of the substrates to the surface of the other substrate, and such heat bonding can significantly change the surface alignment properties of the alignment layer on the two substrates. If a lower temperature heat curing treatment as described above has previously been applied to one of the substrates, for example because the substrate incorporates a colour filter layer, such heat bonding can affect the surface alignment properties of the two alignment layers to different extents, thus producing the undesirable result that the two alignment layers have significantly different surface alignment properties in the manufactured device.
It is an object of the invention to provide an improved method of manufacturing a liquid crystal device, for example by promoting the C2 state in a ferroelectric liquid crystal device during manufacture.